全文获取类型
收费全文 | 3113篇 |
免费 | 47篇 |
国内免费 | 5篇 |
专业分类
化学 | 2161篇 |
晶体学 | 21篇 |
力学 | 42篇 |
数学 | 622篇 |
物理学 | 319篇 |
出版年
2020年 | 22篇 |
2019年 | 27篇 |
2017年 | 22篇 |
2016年 | 66篇 |
2015年 | 41篇 |
2014年 | 56篇 |
2013年 | 129篇 |
2012年 | 100篇 |
2011年 | 102篇 |
2010年 | 75篇 |
2009年 | 75篇 |
2008年 | 121篇 |
2007年 | 95篇 |
2006年 | 107篇 |
2005年 | 84篇 |
2004年 | 99篇 |
2003年 | 73篇 |
2002年 | 74篇 |
2001年 | 44篇 |
2000年 | 39篇 |
1999年 | 63篇 |
1998年 | 49篇 |
1997年 | 43篇 |
1996年 | 53篇 |
1995年 | 41篇 |
1994年 | 41篇 |
1993年 | 42篇 |
1992年 | 41篇 |
1991年 | 48篇 |
1990年 | 48篇 |
1989年 | 49篇 |
1988年 | 57篇 |
1987年 | 55篇 |
1986年 | 43篇 |
1985年 | 62篇 |
1984年 | 68篇 |
1983年 | 58篇 |
1982年 | 75篇 |
1981年 | 59篇 |
1980年 | 54篇 |
1979年 | 51篇 |
1978年 | 60篇 |
1977年 | 52篇 |
1976年 | 46篇 |
1975年 | 56篇 |
1974年 | 41篇 |
1973年 | 27篇 |
1972年 | 34篇 |
1971年 | 32篇 |
1970年 | 38篇 |
排序方式: 共有3165条查询结果,搜索用时 0 毫秒
81.
Sustmann R Sicking W Huisgen R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(10):2245-2255
The mechanisms of cycloaddition of thioformaldehyde S-methylide and thioacetone S-methylide, as models for an alkyl-substituted ylide, to thioformaldehyde and thioacetone, as well as to ethene as a model for a C=C double bond have been studied by ab initio calculations. Restricted and unrestricted B3LYP/6-31G* calculations were performed for the geometries of ground states, transition structures, and intermediates. Although basis sets with more polarization functions were tested, the 6-31G* basis set was applied throughout. Single-point CASPT2 calculations are reported for analysis of the unsubstituted system. The stabilities of structures with high biradical character seem to be overestimated by DFT methods in comparison to CASPT2. The general trends of the results are independent of the level of theory. Thioformaldehyde adds to thioformaldehyde S-methylide without activation energy, and the activation energies for two-step biradical pathways to 1,3-dithiolane are low. C,S biradicals are more stable than C,C biradicals. The two-step cycloaddition is not competitive with the concerted cycloaddition. Methyl substitution in the 1,3-dipole and the dipolarophile does not change the mechanistic relationships. TSs for the concerted formation of the regioisomeric cycloadducts of thioacetone Smethylide and thioacetone were located. Concerted addition remains the preferred reaction. The reactivity of the C=S double bond is high relative to that of the C=C double bond. 相似文献
82.
As a model for the squalene cyclization the interaction between a methyl cation or a methyl radical and two double bonds has been studied using the CNDO/2 and INDO method. In both cases bond formation between the CH3-group and one double bond is facilitated by a second one, but not in a concerted way. 相似文献
83.
Markus Neuenschwander Peter Bigler Klaus Christen Rudolf Iseli Rolf Kyburz Hansueli Mühle 《Helvetica chimica acta》1978,61(6):2047-2058
Chloroacylation and bromoacylation of carbonyl compounds: A forgotten carbonyl reaction. I. Scope of the reaction Aliphatic, α,β-unsaturated and aromatic aldehydes as well as aliphatic ketones react with acyl halides to (α-haloalkyl)esters. These bifunctional derivates contain two leaving groups of different reactivity. The scope of this scarcely of this scarcely known carbonyl reaction is discussed. 相似文献
84.
H. Bartsch H. Bresch M. Gschwendt E. Härle G. Kreibich H. Kübinyi H. U. Schairer Ch. v. Szczepanski H. W. Thielmann und E. Hecker 《Fresenius' Journal of Analytical Chemistry》1966,221(1):424-432
Zusammenfassung Aus Crotonöl konnten durch Anwendung multiplikativer Verteilungs- und chromatographischer Verfahren die entzündlichen und cocarcinogenen Substanzen A1–A4 und B1–B7 rein dargestellt werden. Diese Substanzen sind Diester des polyfunktionellen Diterpenalkohols Phorbol, C20H28O6 mit jeweils einer kurzkettigen (Essig-, (+)-S-2-Methylbutter-, Tiglinsäure) und einer langkettigen Fettsäure (Capryl-, Caprin-, Laurin-, Myristin-, Palmitinsäure). Die Teilsynthese einiger Substanzen wird angegeben. Aus den UV-, IR- und KMR-Daten und chemischen Befunden werden Teilformeln für Phorbol abgeleitet und diskutiert.
Für die Messung von KMR-, Massen- und CD-Spektren und anregende Diskussionen danken wir den Herren Dr. J. Sonnenbichler, München, Dr. J. Jochems, Heidelberg, Dr. A. Mannschreck, Heidelberg, Dr. H. Budzikiewicz, Braunschweig, Dipl.-Chem. C. Wünsche, Heidelberg, Doz. Dr. G. Snatzke, Bonn, und V. Scheidel, Heidelberg.
Vorgetragen von H. Kubinyi. 相似文献
Summary The compounds A1–A4 and B1–B 7 have been isolated from croton oil in pure state by multiplicative distribution and chromatography. These compounds are di-esters of the polyfunctional diterpene alcohol phorbol C20H28O6 each with a short chain (acetic-, (+)-S-2-methyl-butanoic-, tiglic acid) and a long chain fatty acid (octanoic-, decanoic-, dodecanoic-, tetradecanoic-, hexadecanoic acid). The partial synthesis of some compounds is described. From ultraviolet-, infrared-, nmr-spectra and from chemical evidence partial structures for phorbol are being discussed.
Für die Messung von KMR-, Massen- und CD-Spektren und anregende Diskussionen danken wir den Herren Dr. J. Sonnenbichler, München, Dr. J. Jochems, Heidelberg, Dr. A. Mannschreck, Heidelberg, Dr. H. Budzikiewicz, Braunschweig, Dipl.-Chem. C. Wünsche, Heidelberg, Doz. Dr. G. Snatzke, Bonn, und V. Scheidel, Heidelberg.
Vorgetragen von H. Kubinyi. 相似文献
85.
86.
Büttner K Bernhardt J Scharf C Schmid R Mäder U Eymann C Antelmann H Völker A Völker U Hecker M 《Electrophoresis》2001,22(14):2908-2935
Proteomics relying on two-dimensional (2-D) gel electrophoresis of proteins followed by spot identification with mass spectrometry is an excellent experimental tool for physiological studies opening a new perspective for understanding overall cell physiology. This is the intriguing outcome of a method introduced by Klose and O'Farrell independently 25 years ago. Physiological proteomics requires a 2-D reference map on which most of the main proteins were identified. In this paper, we present such a reference map with more than 300 entries for Bacillus subtilis proteins with an isoelectric point (pI) between 4 and 7. The most abundant proteins of exponentially growing cells were compiled and shown to perform mainly housekeeping functions in glycolysis, tricarboxylic acid cycle (TCC), amino acid biosynthesis and translation as well as protein quality control. Furthermore, putative post-translational modifications were shown at a large scale, with 47 proteins in total forming more than one spot. In a few selected cases evidence for phosphorylation of these proteins is presented. The proteome analysis in the standard pI range was complemented by either stretching the most crowded regions in a narrow pH gradient 4.5-5.5, or by adding other fractions of the total B. subtilis proteome such as alkaline proteins as well as extracellular proteins. A big challenge for future studies is to provide an experimental protocol covering the fraction of intrinsic membrane proteins that almost totally escaped detection by the experimental procedure used in this study. 相似文献
87.
88.
Mandy Eibisch Sebastian Zellmer Rolf Gebhardt Rosmarie Süß Beate Fuchs Jürgen Schiller 《Rapid communications in mass spectrometry : RCM》2011,25(18):2619-2626
The liver is an important organ that is particularly involved in the lipid metabolism of the organism. Thus, high interest is nowadays focused on the lipid composition of the liver and particularly the liver parenchymal cells, the hepatocytes. Hepatocytes contain common phospholipids (PL) such as phosphatidylcholines, ‐ethanolamines and ‐inositols, for instance, that can be easily analyzed by matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI‐TOF MS) even without previous separation of the PL mixture. However, in addition to common PL, hepatocytes possess also significant amounts of cardiolipin (CLP). The MS analysis of this PL is quite challenging because it (a) has a higher mass than common lipids and (b) possesses a higher negative charge. We will show here that caution is required if CLP is analyzed directly from the total lipid extract because PC dimers may be interpreted as cardiolipins if the positive ion MALDI mass spectra are analyzed. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
89.
90.
Wolfgang Scherer Prof. Dr. Christoph Hauf Dipl.‐Phys. Manuel Presnitz Dipl.‐Phys. Ernst‐Wilhelm Scheidt Dr. Georg Eickerling Dr. Volker Eyert Dr. Rolf‐Dieter Hoffmann Dr. Ute C. Rodewald Dipl.‐Ing. Adrienne Hammerschmidt Dr. Christian Vogt Dr. Rainer Pöttgen Prof. Dr. 《Angewandte Chemie (International ed. in English)》2010,49(9):1578-1582